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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113779] Very inefficient m68k code generated for simple
 copy loop
Date: Tue, 06 Feb 2024 13:14:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113779

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-02-06
     Ever confirmed|0                           |1

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's already visible with a simple

void f(const long* src, long* dst)
{
  *dst++ =3D *src++;
  *dst =3D *src;
}

where we expand to RTL from

  _1 =3D *src_3(D);
  *dst_4(D) =3D _1;
  _2 =3D MEM[(const long int *)src_3(D) + 4B];
  MEM[(long int *)dst_4(D) + 4B] =3D _2;

there's nothing on GIMPLE that would split the add and RTLs auto-inc-dec
pass doesn't do anything either.  We'd need a form of "strength-reduction"
or maybe targets prefering auto-inc/dec should not legitimize constant
offsets before reload ...

Note with one more copy you then see

  _1 =3D *src_4(D);
  *dst_5(D) =3D _1;
  _2 =3D MEM[(const long int *)src_4(D) + 4B];
  MEM[(long int *)dst_5(D) + 4B] =3D _2;
  _3 =3D MEM[(const long int *)src_4(D) + 8B];
  MEM[(long int *)dst_5(D) + 8B] =3D _3;

and naiively splitting gives you

  src_6 =3D src_4(D) + 4;
  src_7 =3D src_4(D) + 8;

that said, it's really sth for RTL since it's going to be highly target
dependent which form is more efficient.  The auto-inc pass is well
structured, so it should be possible to extend it.=